1. Field of the Invention
The invention described herein relates to time division multiple access (TDMA) communications, and more particularly to synchronization between a headend and remote devices across a TDMA system.
2. Background Art
Certain communication systems include a set of remote communications devices connected to a headend device, such that the headend is responsible for distribution of information content to the remotes. In such a system, the headend may also have administrative functions, such as management of communications between the headend and the remotes. Transmissions from the headend to one or more remotes are denoted herein as downstream transmissions. Transmissions in the opposite direction, from a remote to its associated headend, are denoted herein as upstream transmissions. Because there can be several remotes associated with a single headend, upstream communications must be administered so as to maintain order and efficiency. An adequate level of service needs to be maintained. This can be done through the use of multiple channels in the upstream direction, and the use of time division multiple access (TDMA) communications in each channel of the upstream. In such an arrangement, the upstream bandwidth for each channel is controlled and allocated by the headend. Any given remote can transmit upstream only after requesting bandwidth and receiving a grant of the bandwidth from the headend.
One standard by which such a communications system can operate is the Data Over Cable System Interface Specification (DOCSIS). DOCSIS was originally conceived for cable communications systems. While DOCSIS can be applied to such communications systems, it is not necessarily limited to cable. Wireless communications systems, for example, can also operate under DOCSIS. Likewise, DOCSIS can be used in satellite communications systems.
In the realm of cable communications, DOCSIS specifies the requirements and objectives for a cable headend and for remote cable modems. A cable headend is also known as a cable modem termination system (CMTS). DOCSIS consists of a group of specifications that cover operations support systems, management, data interfaces, as well as network layer, data link layer, and physical layer transport. Note that DOCSIS does not specify an application layer. The DOCSIS specification includes extensive media access layer (MAC) and physical (PHY) layer upstream parameter control for robustness and adaptability. DOCSIS also provides link layer security with authentication. This prevents theft of service and provides some assurance of traffic integrity.
The current version of DOCSIS (DOCSIS 1.1) uses a request/grant mechanism for allowing remote devices (such as cable modems) to access upstream bandwidth. DOCSIS 1.1 also allows the provision of different services to different parties who may be tied to a single modem. With respect to the processing of packets, DOCSIS 1.1 allows segmentation of large packets which simplifies bandwidth allocation. DOCSIS 1.1 also allows for the combining of multiple small packets to increase throughput as necessary. Security features are present through the specification of 56-bit data encryption standard (DES), encryption and decryption to secure the privacy of a connection. DOCSIS 1.1 also provides for payload header suppression, whereby unnecessary ethernet/IP header information can be suppressed for improved bandwidth utilization. DOCSIS 1.1 also supports dynamic channel change. The downstream channel or the upstream channel or both can be changed on the fly. This allows for load balancing of channels and can improve robustness.
In communications systems such as this, propagation delay can be an operational concern and must be accommodated. Any transmission, upstream or downstream, between a headend and a remote device will require some amount of time to reach its destination. Moreover, given a headend and several associated remotes, the upstream propagation delay between each of the remotes and the headend may be different. Efficient upstream communication, however, requires synchronization between a headend and each of its remotes. Contention is minimized and processing is made more efficient if, for example, a headend knows when to expect a transmission from a remote. This is possible only when the headend and each remote have the same sense of time.
DOCSIS provides a solution to the upstream synchronization problem. The headend sends out a synchronization message to all remote devices associated with the headend. The synchronization message contains a 32-bit time stamp, based on 10.24 megahertz (MHz) clock. The time stamp is a statement of the value of the headend's clock at the time of transmission of the synchronization message. The time stamp is used to achieve synchronicity with respect to the upstream communications, by providing each remote with the clock value of the headend, current as of the time of transmission of the synchronization message. Each remote device then locks the frequency and phase of its local clock counter to match the count contained in the received time stamp.
Note that the 10.24 MHz clock can, in some systems, be interpreted in terms of time units, or “ticks.” Each tick can, for example, be 6.25 microseconds. Ticks can be further organized into larger units called minislots. The number of ticks per minislot can be defined at the discretion of the headend. The available upstream bandwidth can therefore be viewed as a series of minislots.
After receiving the time stamp, each remote then adjusts its local clock to compensate for some of the propagation delay between it and the headend. This compensation step takes into account the known factors that contribute to overall propagation delay. Such factors include system topology and downstream interleaving. This compensation is known as ranging offset. Each remote adds the ranging offset to its local 32-bit clock The resulting clock value is then arithmetically converted into a minislot count.
After the synchronization message, the headend sends an initial map message (commonly denoted in its capitalized form, “MAP message” and used hereinafter in this form) to all its remote devices operating on a given channel. This message, in general, tells each remote what minislot(s) the remote can use for transmission in the upstream. This message therefore maps remotes to minislots. This message also defines a specific point in the upstream (e.g., a specific minislot) at which remotes are to respond. When a remote's response is received at the headend, the headend compares the actual arrival time in the upstream with the expected arrival time. Any difference between these two points represents additional (as yet unaccounted for) propagation delay with respect to the responding remote. The headend can then inform the remote of this difference, allowing the remote to further adjust its local clock. As a result of this adjustment, the headend and the remote will have the same sense of time with respect to upstream communications.
In particular, the initial MAP message defines, in the upstream, a starting point (a minislot) in an initial maintenance region (IMR). The IMR represents an interval in the upstream during which any of the associated remotes operating on the given channel can respond to the headend. The MAP message therefore allocates upstream bandwidth. Because the MAP message defines the initial point in time (minislot) in the upstream at which a remote can respond, each remote will respond when its minislot count corresponds to the minislot identified in the MAP message. The headend will then expect a response at that point in time in the upstream.
However, there will typically be some residual propagation delay. The headend will expect a response at a certain point in time in the upstream; the remote will transmit a response at what it believes to be that point in time in the upstream. When the transmission arrives at the headend, it will typically be somewhat later than expected by the headend. This delay represents residual propagation delay of the responding remote. The headend will then tell the remote the size of this residual propagation delay. This allows the remote to further adjust its internal time stamp (TS) counter by this amount. At this point, the remote is effectively synchronized with the headend. A subsequent message sent by the remote at a specific point in time in the upstream will therefore be received at the headend at what the headend understands to be that point in time in the upstream.
Note that the IMR, as allocated by the headend, must be sufficiently large to accommodate any possible propagation delay. As a result, the IMR might represent a significant amount of time (i.e., bandwidth) in the upstream. The IMR must accommodate all of the possible propagation delays for the set of remotes associated with a headend with respect to a particular upstream channel. In some communications systems, however, upstream bandwidth is valuable. It represents opportunities for remotes to transmit information back to the headend. Such transmissions can represent sources of revenue for a communications system provider. Therefore, dedicating a substantial IMR for purposes of achieving synchronization throughout the system represents an inefficiency and a possible loss of revenue to the system provider. Therefore, there is a need for a synchronization process that requires less of the upstream bandwidth for an IMR.